1. Technical Field
The presently disclosed subject matter relates to a long-life hot cathode fluorescent lamp with stable light emission characteristics.
2. Related Art
Cold cathode fluorescent lamps have widely been used as light sources of backlight units for use in large-sized liquid crystal televisions, for example. In light of the recent importance associated with energy savings, it has been suggested that hot cathode fluorescent lamps, superior in light emission efficiency to cold cathode fluorescent lamps, be used as light sources of backlight units.
Light sources of backlight units for liquid crystal display devices have been required to be long life, and small in diameter. These two requirements should be satisfied when hot cathode fluorescent lamps are to be used as light sources of backlight units for liquid crystal display devices.
The lifetime of a hot cathode fluorescent lamp is shortened with degradation of its fluorescent substance, a reduction of filling gas, and other characteristics. The most important determining factor of lifetime is the amount of electron emission material (emitter) that is applied to a coil filament (hereinafter simply referred to as a coil) serving as an electrode. So, a desirable structure of the electrode of a hot cathode fluorescent lamp has been one in which the amount of emitter can be applied can be increased. However, there is a competing interest in that an electrode should also be small in size to allow a small diameter for a fluorescent lamp. Thus, the electrode should be of a structure that allows a greater amount of emitter to be applied in smaller space.
A coil can generally serve as an electrode after being attached to a pair of lead wires and then fixed therebetween. Suppose a case where a coil is incorporated in a hot cathode fluorescent lamp of a large diameter (what is called a fluorescent tube) generally applied for lighting purposes, such as the case disclosed in figure three of Japanese Patent Application Laid-Open No. Hei 6-295704. In this case, the hot cathode fluorescent lamp can have a coil in which the axis of the coil extends in a direction perpendicular to a direction in which the axis of the lead wires extends, and opposite ends of the coil are supported by the tips of the lead wires. Generally, the coil and the lead wires can be fixed by bending and swaging the tips of the lead wires so that the coil can be held in place.
Japanese Patent Application Laid-Open No. 2004-303620 discloses an example of a hot cathode fluorescent lamp of a small diameter. This hot cathode fluorescent lamp employs a structure where the axis of a coil with only one turn extends in a direction perpendicular to a direction in which the axis of a lead wire extends.
Here, it is assumed that a fluorescent lamp with a glass tube of a small diameter employs a structure as disclosed in Japanese Patent Application Laid-Open Nos. Hei 6-295704 and 2004-303620 in which the respective axes of a coil and a lead wire extend in directions perpendicular to each other. In this case, the opposite end portions of the coil may project outward of the lead wire (in the direction of a tube diameter), so that the coil may disadvantageously contact the inner wall of the glass tube.
An electrode structure responsive to reduction of the diameter of a fluorescent lamp and which provides improved means for fixing of a coil and lead wires has been suggested in figure one of Japanese Patent Application Laid-Open No. Hei 6-295704, for example. In this electrode structure, the coil is bent into a U shape, and is then fixed to the lead wires so that the axis of the coil at its opposite end portions extends in the same direction as the axis of the lead wires. Then, the coil is fixed by welding to the lead wires.
Japanese Patent Application Laid-Open No. 2005-235749 discloses a coil formed into a helical shape. Accordingly, in this case, the axis of a filament of the coil at its end portion extends in the same direction as the axis of a lead wire. Thus, the end portion of the coil can be fixed to the lead wire with their respective axes extending in the same direction. Also in this case, the coil and the lead wire are fixed by welding. While not disclosing a concrete way of welding, there is disclosed process steps including welding with the core of the coil remaining as it is, and subsequent dissolving of the core.
When employed in a fluorescent lamp with a glass tube of a small diameter, the above-described lamp structure where the axis of a coil extends in a direction perpendicular to a direction in which the axis of a lead wire extends may cause problems as described below.
In more detail, in the structure of a common hot cathode fluorescent lamp disclosed in Japanese Patent Application Laid-Open No. Hei 6-295704, an end portion of a coil can project outward of a lead wire, so that the end portion of the coil may contact the inner wall of a glass tube (a coil touch is generated). Generation of the coil touch can transfer heat of the coil to the inner wall of the glass tube while the lamp lights up. This melts the glass tube by heating, thereby causing possible leakage, deformation, etc. In a process for activating a material of the emitter applied to the coil by using a coil current to make the material serve as emitter, generation of the coil touch also transfers heat of the coil to the inner wall of the glass tube contacting the coil. In this case, the heat is diffused throughout the glass tube, and then escapes. Accordingly, the emitter cannot be heated and activated sufficiently, thereby shortening the lifetime of the emitter.
Furthermore, using a coil with only one turn as disclosed in Japanese Patent Application Laid-Open No. 2004-303620 results in reduction of coil length. This reduces the amount of emitter that can be applied to the coil, so that attempts to provide a long-life fluorescent lamp are frustrated.
Considered next is a structure where a coil is bent into a U shape and is then fixed to a lead wire as disclosed in Japanese Patent Application Laid-Open No. Hei 6-295704. In this device, when emitter is to be applied in sufficient abundance, a boundary between a region to which emitter is applied and a region to which no emitter is applied is defined in a linear portion near the lead wire. This makes it impossible to define the origin of discharge, resulting in a possibly unstable stage of discharge. Also, in this device, an end portion of the coil and the lead wire are closely aligned along some length, and partially welded by a metal bump at one point. Accordingly, a portion of the coil from its welded point to its end remains unfixed and is a redundant portion that does not contribute to discharge. This redundant portion may disadvantageously be bent to project toward the wall of a glass tube, so that a coil touch may be generated.
Furthermore, the coil is arranged along the lead wire, and is welded to the lead wire at a contact point. This may cause the coil to vibrate easily. Furthermore, the coil, if fixed in a stressed condition to the lead wire, may be deformed while a lamp lights up. Vibration or deformation of the coil that occur while the lamp lights up may make it impossible to achieve desirable characteristics, or may cause a failure that hinders the lamp from lighting up or shortens the lifetime of the lamp.
Also, the coil may be thermally shocked depending on a welding condition, by which tungsten being the principal component of the coil may become brittle. This may cause a failure leading to the wearing out or breakdown of the coil while the lamp in its finished state lights up.
Regarding the helical coil disclosed in Japanese Patent Application Laid-Open No. 2005-235749, a straight filament at an end portion of the coil and the linear lead wire are connected to each other. In order to do so, a connection reinforcing member called a heat tab is used. In this case, the size of the connection reinforcing member may hinder diameter reduction of a glass tube.